One would think that this simple piece of code should compile easily:
#include <utility> struct Q { static const int X = 0; }; int main() { std::pair<int, int>(Q::X, 0); return 0; } However, when I compile this with g++ using the default settings (cygwin gcc 4.5.3) it throws a linker error at me:
undefined reference to `Q::X' I'm totally stumped here -- MSVC compiles this just fine yet my gcc does not. (MinGW also works fine.)
The linker fails to find definition of Q::X.
This is probably because std::pair<> constructor takes arguments as references to const, and Q::X is an l-value, hence it requires an external definition of Q::X to be able to refer to that.
A portable fix:
int const Q::X; 
static const definitions inside a traits(-like) class so it would be quite a burden to duplicate them outside. I think I'll just cast it to a value instead so it doesn't try to take a reference. Thanks for the help!You only declare X but you must also define it, which, for static definitions must happen outside the class
struct Q { static const int X = 0; }; int Q::X = 0; @Maxim Yegorushkin (Beat me to it!) Here is a relevant link that might help.
But your problem is that the int is never externally defined. If X doesn't have to be const, you can initialize it outside the struct in a similar manner. The main reason is because X is static...
struct Q { static const int X; }; int const Q::X = 0; struct Q { static int X; }; int Q::X = 0; 
